With the advent of on-board automotive electronic engine controls, the need to precisely monitor throttle valve position arose. Early throttle position sensors, in their simplest forms, were potentiometers mechanically linked to the throttle valve (i.e. butterfly) shaft. While this solution provided reasonable accuracy and generally adequate performance at a reasonable cost, the nature of the application resulted in unacceptable failure rates.
These failures were caused primarily from the wearing out of the potentiometer hardware. The large number of cycles resulting from normal automobile use, coupled with the "dither" caused by engine vibration, resulted in the contact and/or the resistive element being eroded by friction.
In an attempt to avoid the above-noted problems, non-contact electronic sensors were developed. These attempts, however, were largely unsuccessful, as the high temperature environment of an engine mounted sensor exceeded operational limits of commercial and military componentry. A further complication plaguing most electronic sensors is their inherent temperature instability, which requires extensive and elaborate compensation techniques.
Philips Technical Publication No. 268, entitled "The Magnetoresistive Sensor--a sensitive device for detecting magnetic-field variations", published in the Netherlands and released on Jun. 20, 1988, discloses drive circuitry and an associated coil and sensor arrangement for measuring a magnetic field by a null-field method. An external field is applied to the sensor which causes a current to pass through the coil, thereby setting up a magnetic feedback system in which the coil-generated field almost compensates the external field. A small residual field exists, however, sufficient to maintain the current through the coil and the compensating field generated by it. The sensor output voltage is thus a direct measure of the coil-generated field and hence of the external field to be measured.
U.S. Pat. No. 4,392,375, Eguchi et al., discloses a non-contacting type rotational angle detecting apparatus for sensing the rotational angle of a throttle valve or the like including a magnetic sensing element. The apparatus further includes a non-rotating member fixedly positioned opposite to a rotating member, a magnetic field generating source such as a permanent magnet on one end of the rotating member and the non-rotating member to generate a magnetic field. The magnetic sensing elements generates an output signal which varies in dependence on the direction of the magnetic field. By presetting the intensity of the permanent magnet to a sufficiently large value so as to saturate the output component with respect to the intensity of the magnetic field, variations of the sensor output due to the effect of temperatures can be prevented.
U.S. Pat. No. 4,893,502, Kubota et al., discloses a non-contact type angle sensor for a throttle valve of an internal combustion engine. In one embodiment, the sensor includes a magnetic field generating element disposed on one end of a rotatable throttle shaft, a magneto-sensitive element disposed opposite to the field generating element at a position fixed relative to a throttle body and spaced apart from the field generating element. The magneto-sensitive elements generates an output whose level changes with the rotation of the throttle valve shaft relative to the throttle body and an electrical circuit for converting the output of the element into an electrical signal. In this embodiment, the electrical circuit includes temperature compensation circuitry having an amplifier and a thermistor.
Even those prior art sensors capable of satisfying the temperature and accuracy requirements are often too costly for commercial viability.